fix(protocol): prevent NullPointerException in BpmnElementType

- as the elementTypeName field can be null use Optional.ofNullable in getter

protocol/src/main/java/io/camunda/zeebe/protocol/record/value/BpmnElementType.java

@@ -65,7 +65,7 @@ public enum BpmnElementType {
   }
 
   public Optional<String> getElementTypeName() {
-    return Optional.of(elementTypeName);
+    return Optional.ofNullable(elementTypeName);
   }
 
   public static BpmnElementType bpmnElementTypeFor(final String elementTypeName) {